DESCRIPTION: The relation of magnetic field exposures to melatonin levels in humans will be studied through expanded analysis of an existing data set containing information about magnetic field exposures, visible light exposures, and urinary melatonin in electric utility workers. Data were collected from 192 workers holding a range of jobs at an electric power company. Melatonin was assessed by analysis of 6-OHMS concentration in morning and afternoon urine samples collected over three consecutive nights and work days. EMF and light exposures were assessed by personal dosimetry, with readings every 15 seconds over the 3-day period. Subjects also completed questionnaires and diaries of activities. Three types of analyses will be conducted: 1) Alternative metrics of temporal aspects of EMF exposure, such as its temporal coherence and intermittence, will be examined in relation to nocturnal melatonin. 2) The importance of timing of EMF exposures during the day, relative to melatonin levels at night, will be assessed. 3) Job titles and tasks will be examined to determine if any are particularly associated with altered melatonin levels, and distinctive exposure patterns will be sought for any that are associated.